Final Round of Lens Tests
After the inspiring results with the Schneider enlarging lens, I decided to get my hands on longer focal length enlarger lenses, and also to test the legendary Goerz “Red-Dot” Artar (12-inch f/9), which is apochromatically corrected. Achromatic lenses are designed to focus two wavelengths in the same location (all camera lenses attempt to be at least achromats) whereas apochromats try to focus three. I found a guy on eBay with a remarkable collection and bought a Schneider Componon-S 360mm f/6.8 enlarging lens (so roughly equivalent to the Kodak Commercial Ektar, which is 355mm f/6.3) and a Rodenstock 300mm f/5.6 Apo-Rodagon, the “Apo-”, of course, referring to the fact that it is apochromatically corrected. Recall that since at 1:1 magnification, the film and object are both two focal lengths away from their respective principal planes, the longer focal length should give a much better working distance, with the downfall that the space between the lens and film will have to be folded to make the camera manageable. Perhaps more importantly, the distance between the object and the lens will still be decent at the higher (2 and 3) magnifications.
The principal advantage of the Goerz lens is that it is already f/9, and as we’ve seen in the other tests, none of the lenses so far have performed well at the larger apertures. Presumably if the lens was designed to have f/9 as the largest aperture, the design also has less compromises, since designs for larger apertures should perform at least tolerably well at the large apertures. This gives it a size and weight advantage over the two enormous enlarger lenses. The other advantage is that it is a camera lens, and thus is meant to be assembled around a shutter. Having the shutter at the aperture plane is optimal since as the shutter is opening, it is behaving like a smaller aperture—as opposed to the other extreme of a focal plane shutter which casts a shadow on the film (which is why they are curtains and not diaphragms). This may not be an issue if I get the shutter to fire quickly enough, but it is certainly “nice”. In the case of the enlarger lenses, I at first considered taking them apart and putting a shutter in there, but after seeing them, I think this is a bad idea—the Rodenstock has something over 30 blades in the diaphragm—and I know what a pain it is to put together one with just 5 blades. Thus for those lenses the shutter will have to be outside the lens somewhere. Note that by having it close to the lens it still won’t cast a sharp shadow on the film (so the effect of the images being brighter in the center from the exposure being longer there is not a huge concern) but it has to be slightly larger than it would have to be at the aperture plane. At the aperture, all rays from all object points pass through the same location, so the size can be minimized, whereas anywhere else, the shutter has to increase in size according to the “image cone” of the desired film size. Again, there is not a huge difference, because the lens to film distance is so great, but the difference becomes larger and larger as the aperture decreases in diameter (although this is counteracted by the fact that by shrinking the aperture the shutter has to be smaller anyway). If it turns out the performance of the Goerz is close to that of either of these enlarger lenses (assuming they are superior to the Kodak, of course), then that will be the obvious choice for several reasons.
Essentially the lens decision comes down to this, because this focal length range gives a good working distance at 1:1.
As in the last test, the magnifications are approximate, based on the conjugate distances used. The target was positioned at about 45 degrees to the lens so that one image can also show the depth of field. On to the images, then….
Goerz Red-Dot Artar 305mm f/9, M=1 (working distance ~22 inches)
The Goerz at 1:1 has pretty impressive performance, especially considering the lens I have was made in 1955. I read that the uncoated versions were actually process lenses (designed for 1:1) but after people started using it at other magnifications, Goerz released factory-built lenses with shutters that were optimized for 1:10 (which is what I suspect what my lens is). Evidently 1:1 did not suffer much. (The source of that tid bit is here.) Apparently there are 14″ red dots out there….
But back to the images—note how the focused part of the image remains equally sharp from f/9 to f/22 (there’s slight diffraction blur at f/22), and there’s only a bit of longitudinal chromatic in the out-of-focus regions at f/9, something which disappears by f/22 (almost completely by f/16). (We must remember that the sensor of the D200 I’m using for these tests is much smaller than the intended coverage—6×6 or even 4×5; up to 4×10 or so if I try to do stereo.) In a color photograph, chromatic aberration in the defocused regions may be a problem, but in black and white, it will just add to the blur. Also remember that by the very nature of refraction’s dependence on wavelength, it is impossible to make a lens which does not have chromatic aberration in the defocused regions—different colors of light are taking different paths, but a well-corrected lens will simply have most of the colors meet in a relatively small area on the focal plane for a given magnification.
M=2 (working distance ~15 inches)
At this magnification, in the forward orientation, f/9 through f/22 still work well, but f/11 seems to be the best in the focal region. With the lens backward, the performance seems to improve a bit, back to the condition at M=1 where f/9 through f/16 are nearly indistinguishable, with f/22 adding a bit of diffraction blur.
Rodenstock Apo-Rodagon 300mm f/5.6, M=1 (working distance ~22 inches)
This Rodenstock can be considered to be a newer version of the Goerz, in that they are both apochromats, about 300mm focal length, but the Rodenstock is about 20 years newer. In the forward orientation, there is a bit of chromatic aberration in the focal region even at f/8, but it is gone by f/11, though the sharpest image is at f/16. Diffraction blur proceeds from then on. With the lens backward the performance is very similar (the focal plane evidently was not exactly in the center of the target, which makes it look much worse; it is slightly to the left of the central part).
Head-to-head against the Goerz, the Goerz wins at f/9 (versus the Rodenstock at f/8), performance is almost equal at f/11 (with the Goerz possibly having a slight edge), until at f/16, where the Rodenstock comes out ahead. Since I don’t have a good grasp of the depth of field I’ll need right now, it seems the Goerz is the winner here, on behalf that at f/11 it is not much worse than the Rodenstock at f/16 (in terms of sharpness at least) and the fact that it is smaller and is designed to have a shutter at the aperture plane.
M=2 (working distance ~16 inches)
At this magnification, with the lens forward, there is chromatic aberration near the focal plane even at f/11, but at f/16 and f/22 the image is very nice (with very slight blur at f/22). Perhaps what is surprising is that with the lens backward, the performance is slightly worse. I can’t help but believe it may have to do with my setup, since I’ve had problems before with the camera not being mounted rigidly enough (since it is held by the lens flange on a bellows extension).
But again, the main thing here is, can it beat the Goerz? At f/9, the Goerz has an advantage in the chromatic aberration versus the Rodenstock’s f/8. At f/11, the Goerz seems to have a slight edge in both sharpness and chromatic aberration in the defocused region, and at f/16, the Rodenstock finally seems to overtake its older adversary.
Considering that the Rodenstock performed worse in the backward orientation, while the Goerz performed better, the real comparison should be between the Rodenstock forward and the Goerz backward. As expected, the Goerz is much better at f/9 than the Rodenstock at f/8; Slightly better at f/11, and finally giving way (very slightly) at f/16. Could the Goerz be it?
Schneider Componon-S 360mm f/6.8, M=1 (working distance ~25 inches)
Now for the Componon. Because of its longer focal length, it has a slight working distance advantage over both the Goerz and Rodenstock, which also means my setup gets a little less rigid, and I think in this case there is a bit of motion blur in some of the images.
At this magnification, it seems to work slightly better in the forward orientation. Even at full open, it has impressive sharpness in the focal region (although the exposure is not great in that image). At f/8, the sharpness seems to be comparable to the Goerz’s f/11. At f/16, the two are almost equal, and the Rodenstock wins again.
M=2 (working distance ~18 inches)
Images for the lens forward: f/6.8, f/8, f/11, f/16, f/22, f/32, f/45
Images for the lens backward: f/6.8, f/8, f/11, f/16, f/22, f/32, f/45
(I made a mistake with the files, so there is no image for f/8.)
The Componon seems to perform better backwards here. Unfortunately, those images seem to have a lot of motion blur, and apparently I mixed something up so that f/8 is missing. The critical image at f/16, where the Rodenstock edges out the Goerz, is too blurry to make any sort of judgment. Perhaps it will be worth to repeat the test, if f/16 becomes necessary to get depth of field (remember we still have barely mentioned the needs for a super-fast flash, so the aperture needs to be as big as we can tolerate it).
I think it’s time to do some exposure and depth of field measurements, along with making the choice between 1:1 or 2:1 (though according to these tests it does not dictate the lens choice—which is a good thing: perhaps a convertible camera is possible). If the depth of field is enough at f/11, then it looks as though the Goerz is the way to go. Before diffraction blur starts to affect the image we only have the choice of f/16, and if we have to go there, then evidently the Rodenstock is the winner (though it may be worthwhile to re-test the Schneider).
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- 17.08.07 / 11am